The invention relates to a steering control system for a vessel. In particular the invention relates to a steering control system of a vessel having propulsion units pivotally arranged around an axle which is generally perpendicular to a hull of the vessel, wherein the direction of thrust and thereby the movement of the vessel is controlled by controlling the angular position of the propulsion unit. The invention furthermore relates to the type of propulsion units which are electronically controlled, that is a steering control instrument, for example in the form of a steering wheel or joy sticks, generates input signals to a electronic control unit which in turn controls actuators which turns the propulsion units into a desired position.
Electronically controlled steering systems for vessels are becoming more popular. In electronically controlled steering systems mechanical or hydraulic connections between a steering wheel and the rudder or a pivotally arranged propulsion unit is replaced with an electronic communication channel where input signals from a sensor sensing the position or movement of the steering wheel are transmitted to an electronic control unit controlling actuators which set the position of the rudder or pivotally arranged propulsion unit. An example of an electronically controlled steering system for a vessel is given in WO03/093102. WO03/093102 discloses a steering control system where a steering wheel is coupled to a sensor which senses how far the steering wheel is turned from a starting position. A steering unit receives the input signals from the sensor and generates stored steering angles for the propulsion units. In WO03/093102 the steering unit is arranged to at speed above the hull planing threshold, when running straight ahead, set the underwater housings of the drive units at angle of equal magnitude inclined towards each other, so that the rotational axes of the propellers converge in the forward direction, and to, when turning, the underwater housing closest to the center of the curve is set at a greater steering angle relative to a center plane than the other drive unit. For the purpose of controlling the position of the drive units, the steering unit has stored a fixed value for the toe in position and a fixed ration between the outer and inner drive steering angles for Ackermann steering.
Several problems with known steering systems have been discovered. It has first been noted that vessels are extremely sensitive to the exact position of the propulsion unit when it concerns the roll angle of the vessel and/or lateral forces on the propulsion units. Test have shown that mounting tolerances of a few millimeters may result in that the vessel will obtain an unlevelled roll angle of several degrees when steeling the boat in a straight forward direction. Normally vessel inclination around the length axis of the vessel, that is roll angle position, will be corrected by use of trim planes, which will result in increased fuel consumption or loss of performance. A further problem is known for propulsion units which have a single driving propeller mounted on a propeller axle. This type of propeller generates a reaction force propagating through the propeller axle back up till the engine and the engine mountings. In order to protect the engine mounting from breaking reaction rods may be used. The use of reaction rods has a great impact on the roll angle of the vessel, which is again mitigated by setting of trim planes which will unavoidably result in increased fuel consumption or loss of performance.
Furthermore, the propulsion units are subjected to significant lateral forces from the water flowing by, not only when turning but also when driving straight ahead, where the drive mounting in the hull in particular is subjected to significant stresses, which must be taken into account in the dimensioning thereof. Studies have for example shown, that the waterflow along the bottom of the aft portion of a V-bottomed boat at planing speed is not entirely parallel to the hull bottom. The water flows instead from the center portion of the hull bottom obliquely aft towards the side. Even if the angle is very small, only one or two degrees, the resulting lateral forces on the underwater housing and steering mechanism of the drive units are not negligible.
When turning, the forces on the underwater housing of the drive unit are, of course, larger than when driving straight ahead, especially the forces on the underwater housing of the outer drive unit in relation to the center of the turning curve. On the other hand, the total operating time, during which a boat turns, is relatively small in relation to the time when the boat is moving straight ahead.
A purpose of the present invention is to achieve a method of steering a boat with outboard drive units such that lateral forces having an impact on the propulsion units are controlled. The steering system should for instance ensure that it possible to under straight forward motion of the hull, reduce the forces on the drive units without negatively affecting performance and maneuverability by adding a toe-in or toe-out correction value to a general desired angular position of the propulsion units and to ensure that lateral forces are kept at acceptable levels when turning the vessel, by use of appropriate Ackermann correction values.
It is desirable to provide a steering control system in which the above mentioned problems are solved. According to an aspect of the present invention a steering control system for a vessel includes at least two propulsion units pivotally arranged in relation to the hull of the vessel for generating a driving thrust of said vessel (1) in a desired direction, where the control system includes a steering control instrument for generating input signals for control of a desired route of the vessel a control unit complex controlling the angular position of said propulsion units, said control unit complex being arranged for receiving input signals from said steering control system, which input signals represents a general direction of movement of the vessel and thus a general desired angular position of each propulsion unit said control unit complex furthermore containing a feed forward pivot angle correction control block, which pivot angle correction block is arranged to generate desired angular positions of the propulsion units by adding a correction value to the general desired angular position of the propulsion units. In a preferred embodiment the correction value includes compensation for toe-in or toe-out setting of said propulsion units and/or Ackerman position setting of said propulsion units. That is the steering is performed by to in input signal generated from a steering control instrument, typically a sensor sensing the movement of a steering wheel. The input signal represents a general desired direction of movement. A feed forward pivot angle correction control block is arranged to generate desired angular positions of the propulsion unit by adding a correction value to the general desired angular position of the propulsion units. The pivot angle correction control block is of the feed forward type since it generates desired angular positions of the propulsion units in a feed forward manner by adding correction values to a general desired angular position determined from an input signal generated from a steering control instrument, and which correction values are determined by representations in the form of stored maps or models transforming sensor input signals to a correction value output signal. The correction values typically represent the toe-in or toe-out position and/or the Ackermann position. According to the invention each feed forward pivot angle correction control block is arranged to generate individual correction values for each control unit. Since individual correction angles are generated it is possible to adapt the toe-in or toe-out value for each unit in dependence of the position of the propulsion unit on the hull. It is then possible to set a toe-in or toe-out angle for a specific propulsion unit such that the vessel will not assume an unleveled roll angle when driving in straight forward direction and/or that lateral forces on the propulsion units may deviate form expected values resulting either in excessive wear on the propulsion units or in an increased angular velocity of the propulsion unit when turning, which may result in undesired steering characteristics. That is instead of setting both propulsion units to assume the same toe-in or toe-out angle each propulsion unit is controlled to assume its own unique toe-in or toe-out angle, which may be set for generating a zero roll angle when driving in straight forward direction and/or for generating desired lateral forces on respective propulsion unit. It is furthermore possible to adapt the Ackermann angle to the actual position of the propulsion unit, which is of particular importance when the propulsion units are positioned at different distances from the centerline of the vessel or at different positions along the length axle of the vessel. In the event more than two propulsion units are used or if the propulsion units are asymmetrically positioned with respect to the center line individual setting of Ackermann compensation will be desirable.
In the event any unbalance of the boat exists, such as for example unbalance due to existing reaction rods, or tolerances in the mounting procedure such unbalance can be mitigated by allowing individual correction values for each propulsion unit. In particular it is preferred to set individual toe-in or toe-out compensation values for each propulsion unit for generating a desired roll angle of the vessel or for generating desired levels of the lateral forces when run in forward direction.
Preferably the individual correction values are different for different propulsion units, in particular when the propulsion units are positioned asymmetrically with respect to the center line or in different positions along the length axle of the vessel. Of particular interest is the setting of toe-in or toe-out values and Ackermann values for each propulsion unit. The Ackermann compensation values preferably depend on the position of the propulsion unit in relation to the hull.
The individual correction values for each feed forward pivot angle correction control block are preferably generated by use of in the feed forward pivot angle control block stored maps that for each propulsion unit generates an individual predetermined set correction value dependent on the value of an input signal from a speed control arrangement.
The control unit complex furthermore preferably contains a maximum swing control block, which maximum swing control block is arranged to transform the input signals from said steering control instrument into desired angular positions within an allowed maximum swing range for the propulsion units, wherein the maximum swing control block is arranged to generate individual allowed maximum swing range for each propulsion unit.
Preferably maps stored in the maximum swing control block are used to generate the allowed maximum swing range for each propulsion unit. By use of said maps an individual allowed maximum swing range is set for each propulsion unit, which range is dependent on the value of an input signal from a speed control arrangement.
Generally a common a feed forward pivot angle correction control block can be arranged to determine the individual correction values for each propulsion unit. However it is advantageous to distribute the feed forward pivot angle correction control block into separate control units arranged to each control one propulsion unit. The separate control units receive input signals from a steering control instrument which indicates the desired route of the vessel and locally adapts the pivot angle of the propulsion units by determining the correction values locally. In this embodiment each propulsion unit has its own pivot angle correction control block sub system determining the individual correction values. This idea is generally described in the fourth embodiment disclosed below. It is possible to use the specific features in a central system in a system of having distributed separate control units arranged to each control one propulsion unit.
The invention furthermore relates to a method for operating a steering control system.